1. Field of the Invention
The present invention relates to heat tracing systems for heating pipes, conduits, tanks and like objects using a heat tracing cable.
2. Description of the Prior Art
Electric cable fire alarm systems using heat sensitive conductors were disclosed in U.S. Pat. No. 259,816; 647,556; 664,966; 673,903; 1,235,028; and 2,518,788. These patents were fire alarm systems which used heat sensitive cable to trigger an alarm bell or other device when the ambient temperature was elevated by the occurrence of a fire. These fire alarm systems did not, however, control the supply of heat by the cable to an existing structure, monitor the integrity of the alarm system cable or permit one to locate where there was an abnormally high temperature along the cable.
U.S. Pat. No. 3,814,399 disclosed a control circuit for electric heating blankets wherein current was diverted from the heating coductor through a negative thermal coefficient material to cause the interruption of power to the heating conductor to prevent thermal runaway in the heating conducting elements. This control circuitry required the use of the heating conductor as a part of the control circuit which is undesirable and impractical for heat tracing systems. The control circuitry also required the introduction and maintenance of a voltage phase differential between the heating conductor and the control conductor along the entire length of the conductors. U.S. Pat. No. 3,643,245 disclosed a heat-detecting system which used an elongated negative thermal coefficient thermistor to detect high temperature conditions. However, neither of these patents monitored the integrity of the control circuit or located where the high temperature condition existed in the heat conductor.
Electrical heating cables and tapes, as exemplified by U.S. Pat. Nos. 2,719,902; 3,757,086; 4,072,848; and 4,117,312 have been used commercially for some time to provide heat to pumps and other objects in cold environments. U.S. Pat. Nos. 4,072,848 and 4,112,312 are owned by the assignee of the present application. In the past, control of the temperature of these heating cables was achieved by an external thermostat to interrupt the flow of electrical current to the heating cable at a specified temperature limit or by providing current limiting devices in the heating cable. Externally monitored systems were prone to failure, causing thermal runaway in the heating cable possibly resulting in degredation or destruction of the heating cable. While heating cables provided with integral current limiting control devices had the desirable feature of being positioned coextensively with the actual heater device, a malfunctioning or defective current limiting device could not be detected in advance of a failure, and such systems did not provide an indication of the occurrence or the location of an abnormal temperature condition.